Magnetic stack with oxide to reduce switching current

ABSTRACT

A magnetic stack having a ferromagnetic free layer, a metal oxide layer that is antiferromagnetic at a first temperature and non-magnetic at a second temperature higher than the first temperature, a ferromagnetic pinned reference layer, and a non-magnetic spacer layer between the free layer and the reference layer. During a writing process, the metal oxide layer is non-magnetic. For magnetic memory cells, such as magnetic tunnel junction cells, the metal oxide layer provides reduced switching currents.

RELATED APPLICATION

This application claims priority to U.S. application Ser. No. 12/425,466filed on Apr. 17, 2009 which is based on provisional patent applicationNo. 61/104,400, filed on Oct. 10, 2008 and titled “ST-RAM SwitchingCurrent Reduction Using Free Layer Oxidation”. The entire disclosure ofapplication Ser. Nos. 12/425,466 and 61/104,400 are incorporated hereinby reference.

BACKGROUND

Fast growth of the pervasive computing and handheld/communicationindustry has generated exploding demand for high capacity nonvolatilesolid-state data storage devices and rotating magnetic data storagedevices. Current technology like flash memory has several drawbacks suchas slow access speed, limited endurance, and the integration difficulty.Flash memory (NAND or NOR) also faces scaling problems. Also,traditional rotating storage faces challenges in increasing arealdensity and in making components like reading/recording heads smallerand more reliable.

Resistive sense memories are promising candidates for future nonvolatileand universal memory by storing data bits as either a high or lowresistance state. One such memory, magnetic random access memory (MRAM),features non-volatility, fast writing/reading speed, almost unlimitedprogramming endurance and zero standby power. The basic component ofMRAM is a magnetic tunneling junction (MTJ). MRAM switches the MTJresistance by using a current induced magnetic field to switch themagnetization of MTJ. As the MTJ size shrinks, the switching magneticfield amplitude increases and the switching variation becomes moresevere.

However, many yield-limiting factors must be overcome before suchmagnetic stacks can reliable be used as memory devices or field sensors.Therefore, magnetic stacks with decreased switching current andincreased thermal stability are desired.

BRIEF SUMMARY

The present disclosure relates to a magnetic stack, such as a spintorque memory cell, or magnetic tunnel junction cell, that includes ametal oxide material which is antiferromagnetic at low temperatures andnon-magnetic at high temperatures. The metal oxide layer enablesthermally assisted switching of the magnetization orientation, allowingreduced switching temperature of the magnetic stack.

In one particular embodiment, this disclosure provides a magnetic stackcomprising a ferromagnetic free layer having a switchable magnetizationorientation, a metal oxide layer proximate the free layer, the metaloxide being antiferromagnetic at a first temperature and non-magnetic ata second temperature, a ferromagnetic reference layer having a pinnedmagnetization orientation, and a non-magnetic spacer layer between thefree layer and the reference layer.

In another particular embodiment, this disclosure provides a method offorming a magnetic stack, the method including forming a ferromagneticpinned reference layer, a ferromagnetic free layer, and a non-magneticspacer therebetween, and then forming a metal oxide layer on the freelayer.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIGS. 1A and 1B are cross-sectional schematic diagrams of magneticstacks, in particular, memory cells;

FIGS. 2A and 2B are cross-sectional schematic diagrams of the magneticstack in a first state (FIG. 2A) and in a second state (FIG. 2B);

FIG. 3 is a process flow diagram for a method of making a magneticstack;

FIG. 4 is a schematic diagram of an illustrative memory unit including amemory cell and a semiconductor transistor; and

FIG. 5 is a schematic diagram of an illustrative memory array.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

This disclosure is directed to magnetic stacks (e.g., spin torque memory(STRAM) cells and magnetic read sensors) that include a metal oxidelayer proximate the free layer that at low temperature isantiferromagnetic but at a high temperature is non-magnetic. Byincluding such a metal oxide layer proximate the free layer in amagnetic stack, the thermal stability of the stack can be maintained,and a lower switching current is provided for memory cell embodiments.The metal oxide layer results in reduction of the effective spin torquebarrier due to magnetic moment fluctuation and disorder, thermalassisted switching, and spin specular reflection. The metal oxide layeris easy to control and fabricate, forms a natural interface with thefree layer, and the pinning strength of the metal oxide layer is easilytunable.

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.Any definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

FIGS. 1A and 1B are cross-sectional schematic diagram of a magneticstack 10A. In some embodiments, magnetic stack 10A is a magnetic readsensor such as a magnetic read sensor used in a rotating magneticstorage device. In other embodiments, magnetic stack 10A is a magneticmemory cell 10A and may be referred to as a magnetic tunnel junctioncell (MTJ), variable resistive memory cell, variable resistance memorycell, or resistive sense memory (RSM) cell or the like. Magnetic cell10A includes a ferromagnetic free layer 12 and a ferromagnetic reference(i.e., pinned) layer 14, each having a magnetization orientation.Ferromagnetic free layer 12 and ferromagnetic reference layer 14 areseparated by a non-magnetic spacer layer 13. Note that other layers,such as seed or capping layers, are not depicted for clarity but couldbe included as technical need arises.

Ferromagnetic layers 12, 14 may be made of any useful ferromagnetic (FM)material such as, for example, Fe, Co or Ni and alloys thereof, such asNiFe and CoFe, and ternary alloys, such as CoFeB. Either or both of freelayer 12 and reference layer 14 may be either a single layer or asynthetic antiferromagnetic (SAF) coupled structure, i.e., twoferromagnetic sublayers separated by a metallic spacer, such as Ru orCr, with the magnetization orientations of the sublayers in oppositedirections to provide a net magnetization. Free layer 12 may be asynthetic ferromagnetic coupled structure, i.e., two ferromagneticsublayers separated by a metallic spacer, such as Ru or Ta, with themagnetization orientations of the sublayers in parallel directions. Insome embodiments, ferromagnetic layers 12, 14, particularly free layer12, is formed of a ferromagnetic material with acceptable anisotropy anda saturation moment (Ms) that is at least 500 emu/cc, in someembodiments at least 1100 emu/cc, and in yet other embodiments at least1500 emu/cc, where emu refers to electromagnetic unit of magnetic dipolemoment and cc refers to cubic centimeter.

Either or both layers 12, 14 are often about 0.1-10 nm thick, dependingon the material and the desired resistance and switchability of freelayer 12. In some embodiments, free layer 12 is thinner than referencelayer 14. As will be described below, free layer 12 of the magneticstacks of this disclosure can be thinner than free layers of othermagnetic stacks.

Non-magnetic spacer layer 13 is an insulating barrier layer sufficientlythin to allow tunneling of charge carriers between reference layer 14and free layer 12. Examples of suitable electrically insulating materialinclude oxides material (e.g., Al₂O₃, TiO_(x) or MgO). Non-magneticspacer layer 13 could optionally be patterned with free layer 12 or withreference layer 14, depending on process feasibility and devicereliability.

In the embodiment illustrated in FIG. 1B, proximate ferromagneticreference layer 14 is an antiferromagnetic (AFM) pinning layer 15, whichpins the magnetization orientation of ferromagnetic reference layer 14by exchange bias with the antiferromagnetically ordered material ofpinning layer 15. Examples of suitable pinning materials include PtMn,IrMn, and others. In alternate embodiments, other mechanisms or elementsmay be used to pin the magnetization orientation of reference layer 14.Note that other layers, such as seed or capping layers, are not depictedfor clarity.

In accordance with this disclosure, magnetic stack or cell 10A includesa metal oxide layer 16 proximate ferromagnetic free layer 12. Metaloxide layer 16 is positioned so that free layer 12 is between spacerlayer 13 and metal oxide layer 16. In some embodiments, there is nointervening layer between metal oxide layer 16 and free layer 12.

Metal oxide layer 16 is antiferromagnetic at a first (i.e., low)temperature and is non-magnetic at a second (i.e., high) temperature. Inother words, metal oxide layer 16 has a variable magnetization,depending on its temperature. When metal oxide layer 16 is in itsantiferromagnetic state (see FIG. 2A), magnetic moments 17 of adjacentatoms of layer 16 are ordered and point in opposite directions. Whenmetal oxide layer 16 is in its non-magnetic state (see FIG. 2B),magnetic moments 17 are disordered and randomly fluctuate. At roomtemperature, metal oxide layer 16 is antiferromagnetic, but changes tonon-magnetic at temperatures greater than room temperature, for example,several tens of degrees to a hundred degrees higher. In someembodiments, metal oxide layer 16 is antiferromagnetic at temperaturesless than 200° C. and is non-magnetic at temperatures greater than 200°C. In other embodiments, metal oxide layer 16 is antiferromagnetic attemperatures less than 150° C. and is non-magnetic at temperaturesgreater than 150° C. In still other embodiments, metal oxide layer 16 isantiferromagnetic at temperatures less than 100° C. and is non-magneticat temperatures greater than 100° C.

Examples of suitable materials for metal oxide layer 16 include Co andCo alloy oxides (e.g., CoO_(x)), Fe and Fe alloy oxides (e.g., FeO_(x),FeMnO_(x)), and Ni and Ni alloy oxides (e.g., NiO_(x)) Other metals oralloy oxides may additionally have the required properties, of beingferromagnetic at low temperature and non-magnetic at high temperature.Metal oxide layer 16 may be formed of one or more materials, whicheither individually or together provide the required properties. Ifmetal oxide layer 16 is formed of multiple materials, these materialsmay be present as domains of one material present in a matrix of anothermaterial, or may be stacked layers of materials. For example, metaloxide layer 16 may be a multilayered structure, such as a laminatedmultilayer structure. The material of metal oxide layer 16 is, in mostembodiments, homogenous across its width and length.

Metal oxide layer 16 is often about 5-30 Angstroms thick (e.g., about10-20 Angstroms), depending on the material of metal oxide layer 16 andthe adjacent free layer 12. In some embodiments, metal oxide layer 16may be a non-continuous and/or non-contiguous layer.

To provide current to cell 10A, 10B, a first electrode 18 is inelectrical contact with ferromagnetic free layer 12 and a secondelectrode 19 is in electrical contact with ferromagnetic reference layer14 via optional pinning layer 15. Electrodes 18, 19 electrically connectferromagnetic layers 12, 14 to a control circuit providing read andwrite currents through layers 12, 14.

The resistance across magnetic cell 10A, 10B, and thus data state, isdetermined by the relative orientation of the magnetization vectors ororientations of ferromagnetic layers 12, 14. The magnetization directionof ferromagnetic reference layer 14 is pinned in a predetermineddirection while the magnetization direction of ferromagnetic free layer12 is free to rotate under the influence of spin torque, which may bethe result of current passing through cell 10A, 10B or orthogonal tocell 10A, 10B. During the reading process, which may be with a currentof about 10 μA and at a temperature less than about 90° C. (e.g., about80° C.), metal oxide layer 16 is antiferromagnetic whereas during thewriting process, which is at a much higher temperature (e.g., in someembodiments about 100-200° C.), metal oxide layer 16 is non-magnetic.

In both FIGS. 1A and 1B, the magnetization orientation of free layer 12is illustrated as undefined. In some embodiments, the magnetizationorientation of free layer 12 will generally be either parallel to themagnetization orientation of reference layer 14 (i.e., the lowresistance state) or anti-parallel to the magnetization orientation ofreference layer 14 (i.e., the high resistance state). In someembodiments, the low resistance state may be the “0” data state and thehigh resistance state the “1” data state, whereas in other embodiments,the low resistance state may be “1” and the high resistance state “0”.

Switching the resistance state and hence the data state of magnetic cell10A, 10B via spin-transfer occurs when a current, under the influence ofa magnetic layer of magnetic cell 10A, 10B, becomes spin polarized andimparts a spin torque on free layer 12. When a sufficient level ofpolarized current and therefore spin torque is applied to free layer 12,the magnetization orientation of free layer 12 can be changed amongdifferent directions and accordingly, the magnetic cell can be switchedbetween the parallel state, the anti-parallel state, and other states.In other embodiments of magnetic stacks, the magnetization orientationof free layer 12 is influenced by a neighboring magnetic field, such aslocated on magnetic recording medium. When a sufficient magnetic fieldis applied to free layer 12, the magnetization orientation of free layer12 can be changed among different directions, between the parallelstate, the anti-parallel state, and other states.

The properties of metal oxide layer 16, being antiferromagnetic at lowtemperatures and non-magnetic at high temperatures, allow the use oflower switching current (e.g., no more than about 100-500 μA (about0.1-0.5 mA)) while maintaining thermal stability of the magnetizationorientations, than if no metal oxide layer was present. In someembodiments, the switching current is no more than 400 μA, in otherembodiments no more than about 200 μA. When no switching current ispresent, metal oxide layer 16 is at a low temperature and isantiferromagnetic. The adjacent free layer 12 is stabilized by theantiferromagnetic metal oxide layer 16 due to the exchange coupling atthe interface of free layer 12 and metal oxide layer 16. Because metaloxide layer 16 provides a pinning effect on free layer 12 (whichincreases thermal stability of free layer), a thinner free layer 12 hasthe same level of stability as a thicker free layer with no adjacentmetal oxide layer. In some embodiments, the total thickness of freelayer 12 and metal oxide layer 16 has the same level of stability as athicker free layer with no adjacent metal oxide layer. Because theblocking temperature of the metal oxide is low (e.g., about 100° C., orabout 150° C.), there is no issue regarding temperature variation fromone cell 10A, 10B to an adjacent cell when applying a switching currentto cell 10A, 10B or to the adjacent cell. The antiferromagnetic materialconfines thermal dissipation to the cell being written and thus reducesthe necessary switching current.

When switching current is present for cell 10A, 10B, metal oxide layer16 is at a high temperature and is non-magnetic. The non-magnetic metaloxide layer 16 is less stabilizing to free layer 12 than theantiferromagnetic metal oxide layer 16, thus allowing free layer 12 toreadily switch. Additionally, the non-magnetic property provides a spinspecular (e.g., reflective) effect within free layer 12, thus reducingthe needed switching current for that cell 10A, 10B. A non-continuous ornon-contiguous metal oxide layer 16 may further reduce the switchingcurrent by focusing the current and increasing the current densitythrough free layer 12.

The magnetic stacks of this disclosure, including any or all of layers12, 13, 14, 16, may be made by thin film techniques such as chemicalvapor deposition (CVD), physical vapor deposition (PVD), and atomiclayer deposition (ALD). Metal oxide layer 16 may be formed on free layer12 by oxidizing a portion of free layer 12 or by applying as a separatelayer. For example,

The selection of the materials for metal oxide layer 16 formed fromoxidized free layer 12 depends on the magnetic transport property offree layer 12 (e.g., the magnetic resistance (MR) ratio of free layer 12and its oxide) and the blocking temperature of oxide layer 16. Forexample, CoFeB and NiFeB are preferred free layer materials and theiroxides, CoO and NiO, respectively, have good tenability and reasonableblocking temperatures. CoFe, which has a lower MR, is also acceptable.

FIG. 3 illustrates two methods for forming a magnetic stack having ametal oxide layer. In both methods, the magnetic stack (i.e., includingreference layer 14, spacer 13 and free layer 12) is formed (Step 30).

The first method for forming metal oxide layer 16 includes oxidizingmetal oxide layer 16 (Step 31) by exposing free layer 12 to anoxygen-rich environment. The thickness of the resulting thin layer ofoxide depends on various processing factors such as the level of oxygen,the temperature, and the duration of exposure. In some embodiments, freelayer 12 may be heated to facilitate oxidation. In some methods, theoxidation of free layer 12 to form metal oxide layer 16 is via plasmaoxidation (i.e., using O₂ plasma). The thickness and quality of theresulting thin layer of oxide depends on factors such as O₂ pressure,temperature, and plasma power.

The second method of forming metal oxide layer 16 is to directly form(e.g., deposit) onto free layer 12 (Step 32). The antiferromagneticoxide selected may be different than the ferromagnetic material,allowing for device optimization. For example, a metal oxide layer 16 ofNiO may be deposited onto a free layer 12 formed of CoFeB. Thedeposition of layer 16 may be done, for example, by reactive sputtering.

FIG. 4 is a schematic diagram of an illustrative memory unit 40including a memory element 41 electrically coupled to a semiconductortransistor 42 via an electrically conducting element 44. Memory element41 may be any of the memory cells described herein. Transistor 42includes a semiconductor substrate 45 having doped regions (e.g.,illustrated as n-doped regions) and a channel region (e.g., illustratedas a p-doped channel region) between the doped regions. Transistor 42includes a gate 46 that is electrically coupled to a word line WL toallow selection and current to flow from a bit line BL to memory element41. An array of memory units 40 can be formed on a semiconductorsubstrate utilizing semiconductor fabrication techniques.

FIG. 5 is a schematic diagram of an illustrative memory array 50. Memoryarray 50 includes a plurality of word lines WL and a plurality of bitlines BL forming a cross-point array. At each cross-point a memoryelement 51 is electrically coupled to word line WL and bit line BL.Memory element 51 may be any of the memory cells described herein.

Thus, embodiments of the MAGNETIC STACK WITH OXIDE TO REDUCE SWITCHINGCURRENT are disclosed. The implementations described above and otherimplementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

1. A method of forming a magnetic stack, the method comprising: forming a ferromagnetic pinned reference layer, a ferromagnetic free layer, and a non-magnetic spacer therebetween; and forming a metal oxide layer on the free layer, the metal oxide layer being antiferromagnetic at a first temperature and non-magnetic at a second temperature, the second temperature being greater than the first temperature.
 2. The method of claim 1, wherein forming a metal oxide layer comprises oxidizing a portion of the ferromagnetic free layer.
 3. The method of claim 2, wherein the thickness of the metal oxide layer can be controlled by the level of oxygen present during oxidation of the free layer, the temperature during oxidation of the free layer, the duration of the oxidation of the free layer, or combinations thereof.
 4. The method of claim 2, wherein oxidizing comprises oxidizing in an oxygen rich environment and further comprising heating the free layer.
 5. The method of claim 1, wherein the free layer is oxidized with plasma oxidation.
 6. The method of claim 5, wherein the plasma oxidation is oxygen (O₂) plasma oxidation.
 7. The method of claim 6, wherein the thickness of the metal oxide layer can be controlled by the O₂ pressure, the temperature during plasma oxidation, the plasma power, or combinations thereof.
 8. The method of claim 1, wherein forming a metal oxide layer comprises depositing via reactive sputtering the metal oxide layer on the free layer.
 9. The method of claim 1, wherein the metal oxide comprises a metal element and the ferromagnetic free layer comprises a metal element and the metal elements in the metal oxide and the ferromagnetic free layer are not the same.
 10. A method of forming a magnetic stack, the method comprising: forming a ferromagnetic pinned reference layer, a ferromagnetic free layer, and a non-magnetic spacer therebetween; and forming a metal oxide layer on the free layer, the metal oxide layer being antiferromagnetic at a first temperature and non-magnetic at a second temperature, the second temperature being greater than the first temperature, wherein the metal oxide layer is formed by oxidizing a portion of the ferromagnetic free layer.
 11. The method of claim 10, wherein oxidizing comprises oxidizing in an oxygen rich environment and further comprising heating the free layer.
 12. The method of claim 10, wherein the free layer is oxidized with plasma oxidation.
 13. The method of claim 12, wherein the plasma oxidation is oxygen (O₂) plasma oxidation.
 14. The method of claim 13, wherein the thickness of the metal oxide layer can be controlled by the O₂ pressure, the temperature during plasma oxidation, the plasma power, the duration of oxidation, or combinations thereof.
 15. The method of claim 10, wherein the ferromagnetic pinned reference layer, the ferromagnetic free layer, and the non-magnetic spacer are formed using thin film techniques.
 16. The method of claim 15, wherein the thin film techniques are chosen from chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or combinations thereof.
 17. A method of forming a magnetic stack, the method comprising: forming a ferromagnetic pinned reference layer, a ferromagnetic free layer, and a non-magnetic spacer therebetween; and forming a metal oxide layer on the free layer, the metal oxide layer being antiferromagnetic at a first temperature and non-magnetic at a second temperature, the second temperature being greater than the first temperature, wherein the metal oxide layer is formed by oxidizing a portion of the ferromagnetic free layer using plasma oxidation.
 18. The method of claim 17, wherein the plasma oxidation is oxygen (O₂) plasma oxidation.
 19. The method of claim 18, wherein the thickness of the metal oxide layer can be controlled by the O₂ pressure, the temperature during plasma oxidation, the plasma power, or combinations thereof.
 20. The method of claim 17, wherein the thickness of the metal oxide layer can be controlled by the O₂ pressure, the temperature during plasma oxidation, the plasma power, the duration of oxidation or combinations thereof. 